Unfortunately, a smarter Reddtor GhostOfPluto had to ruin all the fun. As he explains, the elevator is an Otis elevator and:

The Shuffle button is used only by Otis Technicians when they need to change the placement of the lower/upper deck elevators. It allows them to safely board the lower deck car top from the main lobby. A special code is required from the Elevator management system to allow shuffle operation to be activated.” Keep in mind that the elevators in question are double-decked and serve two floors at once.